The present invention relates to an electrolytic display cell and to a process for increasing the storage life of the display. It is intended for use in optoelectronics and particularly the display of alphanumeric characters.
An electrolytic display cell comprises an electrolyte containing a metal salt inserted between an appropriately shaped electrode and a conterelectrode both of which are connected to a power supply able to bring the electrode to a given potential in order to bring about therein either the deposition of a metal layer by electrochemical reduction (display phase), or the re-dissolving of this layer by electrochemical oxidation (erasure phase).
One of the major problems caused by such devices is the life of the cell. Maximum stability of the substances present is necessary to obtain a long life.
The stability of the electrolyte can be obtained by using an organic solvent and a salt, which is such that secondary reactions which may appear at the end of dissolving are reversible, so that the product produced by the secondary reactions decomposes again into its initial components. The composition of the electrolyte then remains substantially unchanged. The salt is generally a metal halide, particularly silver bromide or idodide.
However, there is also a difficulty linked with the duration of the storage phenomenon of the displayed symbols. A storage time of approximately 10 minutes can be obtained with such electrolytes (e.g. with silver iodide). However, a problem occurs in connection with the maintainance of this time following display "refreshing" cycles.
As is known this consists of an erasure and re-writing operation carried out as quickly as possible in order to regenerate the display contrast. The period of time between two refreshing operations is dependent on the ability of the cell to store the display when excitation is at an end. It rises with an increase in the storage time. A period of 10 minutes would appear to constitute a minimum in applications to clocks and watches, so that power consumption remains limited.
To give a better understanding of the difficulties which may occur during such a refreshing operation it is necessary to briefly describe the phenomena leading to a deterioration of the storage. These phenomena are of two different types, depending on whether they relate to excess erasure or a lack of stability of the structure of the metal film. During an excess of erasure a large quantity of halogen (iodine for example) is formed in the vicinity of an erased segment. If the latter is immediately re-written the halogen which has not had time to diffuse dissolves part of the silver which has just been deposited, leading to a reduction in the contrast. The reduction in the optical properties of the silver film (and particularly its absorption) is linked with a development of the structure of the film during the storage period. Whereas initially the structure was monocrystalline and very absorbent for optimum deposition conditions it develops towards an increasingly less absorbent filamentary form. Thus, the contrast is progressively reduced to zero. Moreover, for a filamentary surface, during the dissolving of the last silver filaments, iodine simultaneously forms on the remainder of the film surface. Thus, the iodine quantity formed is greater than in the case where the structure is microcrystalline. Finally, if an erasure has been inadequate the metal layer develops more rapidly in the following writing cycle because the undissolved silver grains serve as recrystallization nuclei, so that the storage progressively deteriorates.
Satisfactory operation can only be obtained if it is possible to perfectly redissolve the silver film without the iodine quantity formed being too great. For this purpose it is necessary that the development of the film structure has been sufficiently small, i.e. that it is stable.